Tuning element for a cavity resonator



28, 1956 D. Q. POSIN 2,761,106

TUNING ELEMENT FOR A CAVITY RESONATOR Filed Feb. 5, 1946 INVENTOR.

DANIEL Q. POSlN ATTORNEY United rates Patent TUNING ELEMENT FOR A CAVITY RESONATOR Daniel Q. Posin, Cambridge, Mass., assignor, by mesne assignments, to the United States of America as repre sented by the Secretary of the Navy Application February 5, 1946, Serial No. 645,625

8 Claims. (Cl. 333-83) This invention relates to high frequency capacitative structures and more particularly to variable capacitative devices employed for the purpose of tuning cavity resonators.

The tuning device described herein is an improvement on the tuning device described and claimed in the application of Persa it. Bell, Serial No. 640,448, filed January 11, 1946, now Patent No. 2,736,868.

Heretofore capacitative structures for cavity tuning have been constructed but have not been capable of producing a constant output over a wide frequency range.

A specific object of this invention is to provide a tuning element capable of varying the frequency of a cavity resonator over a wide range of frequencies at a constant output.

Another object is to provide a tuning element for accomplishing the above object which is both simple and susceptible to calibration.

A further object is to provide a tuning element which can easily be adapted to the wobbulation of the frequency of a cavity resonator.

A still further object is to provide a tuning element which is simple both in construction and in operation.

With these objects in view, a cavity tuning element has been devised comprising a dielectric rod slidably inserted into an opening provided in a wall of a cavity to be tuned, a small metallic fitting, capacitive in nature, being mounted on that end of the rod disposed within the cavity. The resonant frequency of the cavity is adjusted by the depth of insertion and the angle of rotation of the capacitive fitting.

The structural details and features of the present invention are described in the following specification and shown in the accompanying drawings of which:

Fig. 1 is a perspective view of one form of the invention;

Fig. 2 is a plan view of part of Fig. I;

Fig. 3 is a cut away view of one form of the invention;

Fig. 4 is a cut away view of the structure of the invention inserted in the resonant cavity of a high frequency oscillator;

Fig. 5 is another sectional view taken along the line V--V of Fig. 4.

Broadly, the invention involves the insertion of a thin U-shaped strip of metal or metal foil into the external cavity of a cavity resonator to change the distributed capacitance and in turn the resonant frequency of the cavity. The U-shaped strip of metal is widest at the open ends of the U and tapers gradually and uniformly to a minimum width at a point located at the vertex, which is the mid-point, at the base of the U, between the open ends of the M-shaped member.

It has been determined experimentally that a frequency variation of the order of ten percent is attainable at a substantially constant power output by a cavity tuner having the shapes hereafter disclosed. While cavity tuners having a U-shaped configuration have heretofore been designed, such tuners are capable of attaining only a frequency variation of the order of two and one half percent at a substantially constant power output. In addition, the frequency modulation achieved by the present cavity tuner is more uniform than that attained by previous U-shaped tuners. The reason for this will become apparent from the more detailed description which follows.

Referring to Fig. 1 there is shown a U-shaped fitting 10 of copper or silver mounted on polystrene block 11. Fitting 10 consists of cylindrically curved walls 12 joined by a curved end piece 13 of lesser width than walls 12.

Referring to Fig. 2 the construction of fitting 10 is shown. It is in the dimensions of fitting 10 that lie the novel features of the present invention. The ratio of the Width at 13 (the mid-point of the base of the U) to the width at 12 (the open ends of the U) is very critical if constant output is to be maintained. An experimental value of five to eight has been found to be satisfactory. More specifically, if A be designated a unit of measurement and the length of the member before being bent to a U-shape is designated by a convenient unit of measurement 2A, then the width of the member at the open ends will be equal to one-quarter A. As shown, the member is uniformly tapered along its length beginning from points at a distance from each end equal to five sixteenths A, the radius of the taper in each edge of the member being such as to provide a width of five thirty-seconds A at the mid-point between the open ends. It will readily be understood that this mid-point will form the extreme bottom of the U when the member is formed into a U-shape.

Referring to Fig. 3 there is shown a more secure Way of securing fitting 10 to rod 11. Herein a recess 14 is made in an end of rod 11, said recess 14 being cylindrically shaped to receive the fitting 10 as shown.

Fig. 4 is a cut away view showing the tuning structure of Fig. 3 inserted into the cylindrical resonant cavity 13 of a McNally oscillator.

Fig. 5 is another sectional view of the same structure. As is well known, the cavity 18 of a McNally oscillator is external to the glass seals 15 and is electrically connected to the internal structure of the tube by metallic discs 16.

A passage is drilled radially into the cylindrical cavity 18 midway between the top and bottom of said cavity, this passage being large enough to receive the rod 11. Any number of well known supporting and clamping structures may be employed to hold and allow positioning of the rod 11.

As is clearly shown in Fig. 5 the walls 12 of the fitting 10 form two cylindrical capacitors with the surfaces 19 of the cavity 18. These two capacitors are connected efiectively in series by the end piece 13 .of the fitting ltl (Fig. 3) and their magnitude is controlled by the depth of insertion and the angle of rotation of the fitting lb. The end piece 13 of the fitting 10 contributes an inductive effect which will resonate with the distributed capacitance in the cavity.

The dimensions of the end piece 13 of the fitting 10 are critical if a Wide band width is to be attained. The Q of a circuit is well known in the art as the determinant of bandwidth where W=2vrxfrequency =inductance R=resistance In order to get a lower Q and thus a lower bandwidth, it is necessary to increase R or decrease L. This is efiectively accomplished by decreasing the size of end piece 13 to a smaller cross section than that of the outer ends 12. The decrease in area provides less surface area to form an electric field and act as an inductance. The decrease of cross sectional area also causes end piece 13 to have a greater resistivity. The combination of the two factors combine to produce a greater bandwidth. The rod 11 functions primarily as the position controlling means of the tuning element, but with the structure of Fig. 3, also serves as the dielectric medium of the two variable capacitors formed.

In the apparatus of Figs. 4 and 5 the resonant fre quency of the cavity 18 is lowered by the introduction of the tuning element of this invention. Maximum tuning elfect is had when said element is fully inserted and when angularly positioned as shown in Fig. 5.

A form of the invention comprises a cavity resonator having a multiplicity of capacitive tuning devices of the type shown in Fig. 1. The tuning means of this invention is obviously not limited to application to McNally cavities alone.

It will be apparent to those skilled in the art that the resonant frequency of the cavity structure shown in Fig. 4 may be wobbulated in any number of ways such as by continuous or back and forth rotation of the rod 11 about its longitudinal axis, or by an in and out motion of the rod 11 relative to the cavity 18. It can also be seen that the tuning structure of Fig. 3 can be calibrated in terms of frequency.

The invention described in the foregoing specification need not be limited to the details shown, which are considered to be illustrative of only one form the invention may take. What I desire to secure by Letters Patent and claim is:

1. In combination, a cavity resonator having a pair of end walls and a conducting U-shaped member haying two substantially parallel side pieces and a curved end piece joining said side pieces, said side pieces being diametrically opposite each other and having a portion of each edge thereof uniformly tapered to provide a minimum width at the base of said end piece, and a dielectric rod fastened to the open ends of said U-shaped member, said U-shaped member being disposed within said resonant cavity and adapted for rotational movement with respect to said end walls thereby to vary the frequency to which said resonant cavity is tuned.

2. In combination, a cavity resonator having a pair of end walls and a substantially U-shaped conductive member having two substantially parallel sidepieces and a curved endpiece joining said sidepieces, said two sidepieces being curved to conform to a common cylindrical surface, said two sidepieces occupying diametrically opposite positions on said cylindrical surface, said U-shaped member it developed into a fiat sheet having the form of a substantially rectangular conductive strip having a width that is a small fraction of its length and a thickness small compared to its width, the center portion of said strip being arcuately tapered in width to a minimum width midway the ends of said strip, and a dielectric rod, said U-shaped conductive member being joined to an end of said dielectric rod at the open end of said U-shape with the axis of said cylindrical surface coincident with the axis of said rod, said U-shaped conductive member being disposed between said end walls and adapted for rotational movement so as to tune said resonant cavity through a relatively wide band of frequencies.

3. In combination, a cavity resonator having first and second spaced end walls and a substantially U-shaped conductive member having two substantially parallel sidepieces and a curved endpiece joining said sidepieces, said two sidepieces being curved to conform to a common cylindrical surface, said two sidepieces occupying diametrically opposite positions on said cylindrical surface, said U-shaped member if developed into a flat sheet having the form of a substantially rectangular conductive strip having a width that is a small fraction of its length and a thickness small compared to its width, the center portion of said strip being arcuately tapered in width to a minimum width midway the ends of said strip, said minimum width being substantially five-eighths the width of said strip at the ends thereof, and a dielectric rod, said U-shaped conductive member being joined to an end of said dielectric rod at the open end of said U-shaped member with the axis of said cylindrical surface coincident with the axis of said rod, said U-shaped conductive member being mounted for rotational and translational movement with respect to said end walls, thereby to change the frequency at which said cavity resonates.

4. In combination, a cavity resonator having a pair of spaced end walls and a substantially U-shaped conductive member having two substantially parallel sidepieces and a curved endpiece joining said sidepieces, said two sidepieces being curved to conform to a common cylindrical surface, said two sidepieces occupying diametrically opposite positions on said cylindrical surface, said U-shaped member if developed into a fiat sheet having the form of a substantially rectangular conductive strip having a width that is a small fraction of its length and a thickness small compared to its width, the center portion of said strip being symmetrically and arcuately tapered in width to a minimum width midway the ends of said strip, and a dielectric rod, said U-shaped conductive member being joined to an end of said dielectric rod at the open end of said U-shaped member with the axis of said cylindrical surface coincident with the axis of said rod, and means for retaining said U-shaped oonductive member between said end walls such that said sidepieces and said walls form a pair of capacitors connected in a series relationship by said curved endpiece, said last-mentioned means permitting rotational movement of said U-shaped conductive member whereby the value of said capacitors may be varied to effectively change the tuning of said cavity resonator.

5. In combination, a cavity resonator having two spaced end walls and a substantially U-shaped conductive member having two substantially parallel sidepieces and a curved endpiece joining said sidepieces, said two sidepieces being curved to conform to a common cylindrical surface, said two sidepieces occupying diametrically opposite positions on said cylindrical surface, said U-shaped member if developed into a flat sheet having the form of a substantially rectangular conductive strip having a width that is a small fraction of its length and a thickness small compared to its width, the width of a portion of said strip substantially three-quarters the length of said strip and disposed midway the ends of said strip being symmetrically and arcuately tapered in width to a mini mum width midway the ends of said strip, said minimum width being substantially five-eighths the width of said strip at the ends thereof, and a dielectric rod, said U- shaped conductive member being joined to an end of said dielectric rod at the open end of said U-shaped member with the axis of said cylindrical surface coincident with the axis of said rod, said U-shaped conductive member being disposed within said cavity resonator such that said sidepieces and said end walls cooperate to form variable capacitors, the values of which may be adjusted by varying the angular position of said member with respect to said end walls.

6. In combination, a cavity resonator ha ing two spaced end walls and substantially U-shaped conductive member having two substantially parallel sidepieces and a curved endpiece joining said sidepieces, said two side pieces being curved to conform to a common cylindrical surface, said two sidepieces occupying diametrically opposite positions on said cylindrical surface, said U-shaped member if developed into a fiat sheet having the form of a substantially rectangular conductive strip having a width at either end thereof equal to one-eighth of its length and a thickness small compared to its width, each longer side of said strip being arcuately tapered between points substantially & of the length of said strip from either end of said strip such that said strip has a minimum width midway the ends thereof, said minimum width being substantially of the length of said strip, and a dielectric rod, said Ushaped conductive member being joined to an end of said dielectric rod at the open end of said U-shaped member with the axis of said cylindrical surface coincident with the axis of said rod, means for securing said U-shaped conductive member within said cavity resonator with said sidepieces cooperating with said end walls to form a pair of variable capacitors connected in a series electrical relationship by said curved endpiece, said last-mentioned means permitting relative rotational and translational movement between said U-shaped member and said end walls whereby the value of said capacitors may be altered to tune said resonant cavity through a wide frequency band.

7. In combination, a cavity resonator having two spaced end walls and a substantially rectangular conductivc strip having a width at either end thereof equal to one-eighth of its length and a thickness small compared to its width, each longer side of said strip being arcuately tapered between points substantially of the length of said strip from either end of said strip such that said strip has a minimum width midway the ends thereof, said minimum width being substantially of the length of said strip, said strip being formed into a U-shaped member having two substantially parallel sidepieces and a curved endpiece joining said sidepieces, said two sidepieces being curved to conform to a common cylindrical surface, said two sidepieces occupying diametrically opposite positions on said cylindrical surface, a dielectric rod, said U-shaped conductive member being joined to an end of said dielectric rod at the open end of said U with the axis of said cylindrical surface in coincidence with the axis of said rod, and means for retaining said U- shaped member within said cavity resonator with said parallel sidepieces cooperating with adjacent end walls to form a pair of capacitors connected in a series relationship by said curved endpiece, said last-mentioned means permitting rotational and translational movement of said U-shaped member with respect to said end walls whereby the magnitude of said capacitors is altered and whereby the tuning of said cavity resonator is changed.

8. In combination, a cavity resonator having two spaced parallel end plates, a portion of each of said end plates being formed to conform to the surface of a circular cylinder, said cylinder having an axis extending between said plates and substantially parallel thereto, and a tuning element comprising a substantially rectangular conductive strip having a width at either end thereof equal to one-eighth of its length and a thickness small compared to its width, each longer side of said strip being arcuately tapered between points substantially of the length of said strip from either end of said strip such that said strip has a minimum width midway the ends thereof, said minimum width being substantially of the length of said strip, said strip being formed into a U-shaped member having two substantially parallel sidepieces and a curved endpiece joining said sidepieces, said sidepieces and said cylindrically formed portion of said end plates being disposed in spaced juxtaposition.

References Cited in the file of this patent UNITED STATES PATENTS 2,126,868 Brown Aug. 16, 1938 2,240,382 Van Alstine Apr. 29, 1941 2,280,824 Hansen Apr. 28, 1942 2,311,520 Clifiord Feb. 16, 1943 2,442,671 Tompkins June 1, 1948 

